In accurate position detecting techniques such as a stepper for synchrotron radiation lithography, or a photo-stepper, an optical heterodyne position detecting system has been put to practice to trial machines, for example, Japanese Patent Laid-Open 62-261003 or same 64-89323.
FIG. 25 schematically shows a position detecting device which makes use of interference of diffracted beams in the optical heterodyne method shown in the former of said prior art.
This device is composed of respective diffraction gratings 1a, 1b formed on a mask A which is a first substance and on a wafer B which is a second substance; a light source 2 comprising a transverse Zeeman laser; incident angle adjusting means comprising mirrors 30, 31 which adjust directions of light separated by a later stated polarizing beam splitter, and inject the light from a direction of angle of .+-..theta.n (.theta.n satisfies n.multidot..lambda.=P.multidot.sin .theta.n which is a formula of diffraction where .lambda. is wavelength of the light source, and n is a positive integer) with respect to normal lines of the grating faces of the diffraction gratings 1a, 1b; a light interference means comprising a polarization beam splitter 5 which separates two polarized beams from the light source 2 so as to branch them toward the mirrors 30, 31 respectively, and polarizing plates 58, 59 which cause the diffracted lights taken out from said diffraction gratings 1a, 1b in perpendicular directions with respect to the diffraction gratings to cause interfere each other; a detecting means comprising detectors 604a, 605a which detect beat signals generated by the interference of the light from the polarizing plates 58, 59; and a signal processing means 7 which detects phase differences of the beat signals detected by said detectors 604a, 605a.
The light emitted from the source 2 has two frequency components f1, f2, and is divided, via the polarization beam splitter 5, into a light of the frequency component of f1 and a light of the frequency component of f2, and these lights which are orthogonally polarized go into the diffraction gratings 1a, 1b from the .+-.n-th order directions (e.g., .+-.1st order directions) by the mirrors 30, 31. The lights f1, f2 (shown with dotted lines) diffracted in said perpendicular directions from the diffraction gratings 1a, 1b pass through a mirror 203 and a prism mirror 207, and are made coherent at the respective polarizing plates 58, 59, and the beat signals are detected by the detectors 604a, 605a. Between said detected beat signals, phase differences occur in proportion to amounts of positional shifts of the diffraction gratings 1a, 1b. By detecting said phase differences through a phase detector of the signal processing means 7, the amount of the relative positional shifts between the two diffraction gratings 1a, 1b may be conceived.
In the latter technology (64-89323), the above mentioned injection angle adjusting means has been changed, in which the light is caused to be injected to the diffraction gratings at desired angles through a reference grating and a Fourier transforming lens. As shown in FIG. 26, the coherent light emitted from the light source 2 is injected to the reference grating 38 and generates a diffracted light. The Fourier transforming lens 39 composed of a special filter and a pair of lenses selectively passes only such lights diffracted in .+-.m-th order directions (m is a positive integer) among the lights diffracted by the reference grating 38, and such diffracted lights are injected from the .+-.n-th order directions to the both diffraction gratings 1. In the same, the reference numeral 56a designates a half wave plate which rotates by 90.degree. the polarizing condition of either of the lights diffracted by the reference grating 38, so that when the diffracted lights are re-diffracted later, this re-diffracted lights can interfere.
In the above mentioned optical heterodyne position detecting system, the larger are the grating constants of the diffraction gratings 1a, 1b, i.e., grating pitches P, or reversely the smaller are absolute values n of the order in the injecting direction of the coherent lights, the wider becomes the signal detecting range. The relation therebetween is the signal detecting range=P/2n. For example, when n is 1, 1/2 of said pitch P falls within the detecting range.
However with respect to a detecting resolution, an absolutely reverse relation thereto is established. If accuracy of the resolution of the phase detector is assumed to be 1.degree., the detecting resolution is the detecting resolution=the signal detecting range/360.degree.. So far as the absolute value n of the order in said light injecting direction is not increased, the larger is the grating pitch P, the lower becomes the accuracy of said detecting resolution. Therefore if trying to obtain the diffraction gratings 1a, 1b having the small grating pitches P for heightening the detecting resolution, the signal detecting range is made extremely narrower according to the above mentioned relations (when the absolute value n of the order in the injecting directions of the coherent lights are enlarged to heighten the detecting resolution, the same results are obtained as the above relation).
Therefore if trying to obtain a higher position detecting resolution by means of such an optical heterodyne position detecting system in the stepper for a synchrotron radiation lithography of a quarter micron level, the signal detecting range becomes very narrow. Thus, it has been difficult to put the conventional system to practice.